1. Field of the Invention
This invention relates to modifying zeolite minerals in order to increase their affinity for arsenic species, and their use in removing arsenic species from an aqueous medium.
2. Description of the Background Art
Arsenic is a shiny, gray, brittle element possessing both metallic and non-metallic properties. It is stable in its elemental form, but is most commonly found in the trivalent form (Arsenite) and pentavalent form (Arsenate). Its compounds may be organic or inorganic. The dominant forms of arsenic present in natural waters at common pH (6-8) are the monovalent H.sub.2 AsO.sub.4.sup.- and the divalent HAsO.sup.2- forms of arsenate and the uncharged form of arsenite, arsenious acid HAsO.sub.2.
Inorganic arsenic is highly toxic to mammals and aquatic species. When ingested, it is readily absorbed from the gastrointestinal tract, the lungs, and to a lesser extent from the skin, and becomes distributed throughout the body. Trivalent inorganic forms of arsenic are more toxic than the pentavalent forms. Recently, arsenic in water supplies has been linked to arsenical dermatosis and skin cancer.
Naturally occurring ubiquitous arsenic is present in the environment and makes of 0.00005% of the earth's crust. Hence it is found in trace quantities in many ground and surface waters. However, arsenic has many industrial uses such as hardening of copper and lead alloys, pigmentation in paints and fireworks, and the manufacture of glass, cloth, and electrical semiconductors. Arsenic is also used extensively in the production of agricultural pesticides, which includes herbicides, insecticides, desiccants, wood preservatives and feed additives. Runoff from these uses and the leaching of arsenic from waste generated from these uses have resulted in increased levels of various forms of soluble arsenic in water. Because of recent studies further revealing its toxicity, the United States Environmental Protection Agency (EPA) has classified arsenic as a human carcinogen (Group A) and is considering lowering its maximum contaminant level from its present requirement 50 parts per billion (ppb) to 5 ppb or less.
In order to keep the water supply safe for human consumption, and affordable to all, water utilities are presently examining methods of treating water in order to reduce levels of arsenic in the water to 5 ppb. One such method is adsorption, which is the bonding of an aqueous species to the surface of a solid grain. The solid grain is called the sorbent while the aqueous species is called the sorbate. The nature of the sorbent, including functional groups and the surface area available for adsorption, affects the affinity of the sorbent for specific contaminants. Also, the chemical character, shape, and configuration of the sorbate, its water solubility, its acidity, the polarity of the molecule, its molecular size and polarizability all affect its ability to sorb onto the reactive media.
One method for removing arsenic species from an aqueous medium is through the use of an alumina sorbent. U.S. Pat. No. 5,556,545 (Konstantin, et al.), which is incorporated by reference herein, discloses the use of activated alumina sorbent having a particulate size below 200 micrometers diameter and with sufficient porosity and pore diameters above 100 Angstroms to remove arsenic from water. This method is done using a slurry of the activated alumina sorbent and water. After a certain period of time, the activated alumina sorbent is removed and the water is recovered. The spent activated alumina sorbent is then regenerated and recycled.
However, the use of activated alumina as sorbent contains some inherent limitations. For example, in order to make the removal of arsenic with activated alumina sorbent an economically feasible process, rejuvenation and conditioning of the sorbent for subsequent use is necessary. Rejuvenation and conditioning of the sorbent is a process wherein the sorbent is made to release its adsorbed arsenic. Therefore, this rejuvenation process creates a hazardous solution which requires further treatment and ultimately expensive disposal costs.
Another limitation connected with the use of activated alumina as a sorbent to remove arsenic species from water involves the regeneration of the sorbent. More specifically, some sorbent is lost in every regeneration due to the strong alkaline solution necessary to remove the adsorbed arsenic. Hence, lost activated alumina sorbent must continuously be replaced which substantially increases the cost of using activated alumina as a method for removing arsenic from an aqueous medium.
Another method for removing arsenic species from an aqueous medium is through the use of activated carbon. Activated carbon is available in powdered (PAC) and granular (GAC) forms. The powdered form is generally utilized in a batch process, most often in conjunction with another unit process. Studies have shown that the addition of a powdered activated carbon to a lime softening process can enhance arsenic removal (Dutta, A. and M. Chaudhuri. "Removal of arsenic from groundwater by lime softening with powdered coal additive." Aqua, vol. 40, no. 1 (1991) pp. 25-29). Lime softening and PAC alone were found to remove 90% and 15%, respectively of the aqueous arsenic species present.
However, the use of activated carbon to remove arsenic species from an aqueous medium has inherent limitations in that activated carbon has a limited natural capacity for adsorbing arsenic species. Further, activated carbon has a high cost making it less attractive as a chosen method for removing arsenic species from an aqueous medium.
Yet another method for removing arsenic species from an aqueous medium is through the use of fly ash. Fly ash is a waste product produced in large quantities at coal power stations. It is composed primarily of calcium oxide, CaO, but also may contain magnesium, aluminum and iron oxides.
However, the properties of fly ash produced by a particular power station are dependent upon the fuel used at that power station. Hence, quality control and the fly ash's capacity for arsenic species are difficult to maintain. Moreover, fly ash is produced only in a powdered form, and therefore has limited application in column separation.
Still another presently known method to remove arsenic species from an aqueous medium is ion exchange. Ion exchange is essentially a sorption process. Arsenic anions enter the exchanger structure and replace labile anions. One of the disadvantages of this process is that the ion-exchangers utilized are mostly synthetic resins and hence are expensive. Further, few arsenic selective resins presently exist. Consequently, ubiquitous anions such as sulfates compete for the ion-exchange sites in the resin. Chromatographic peaking of toxic arsenic levels can occur in this situation.
In general, ion-exchange is not a feasible method of removing arsenic from an aqueous medium if the medium contains a high level of dissolved solids or sulfate concentrations. Ion exchange resins are generally regenerated with a sodium chloride solution. As is the case with activated alumina, the spent regenerant will require treatment prior to reuse or disposal.
Another method for removing arsenic species from an aqueous medium is through the use of a membrane process. A membrane process involves passing the aqueous medium through the membrane to filter the selected material. For example, reverse osmosis has been shown to reject arsenic in commercial and household (point of use treatment) applications (please see Connell, P. J. and T. A. Marr. "Emergency response spill cleanup of wood treating waste." Water Pollution Research Journal of Canada, vol. 25, no. 3 (1990), pp. 265-273; Fox, Kim R. and Thomas L. Song. "Controlling arsenic, fluoride and uranium by point-of-use treatment." Journal of the American Water Works Association (1987); Fox, Kim R. "Field experience with point-of-use treatment systems for arsenic removal." Journal of the American Water Works Association, vol. 81, no. 2 (1989), pp. 94-101; Rozelle, Lee T. "Point-of-use and point-of-entry drinking water treatment." Journal of the American Water Works Association (1987); Stass, A. A. "Osmose water purification system to remove CCA contaminants from water." Arsenic and mercury workshop on removal, recovery, treatment, and disposal, EPA/600/R-92/105. U.S.E.P.A., Cincinnati, (1992), pp. 30-32). Discharge levels of 0.05 mg/l have been met.
However, membrane processes are similar to the ion exchange process described herein due to the fact that they both require treatment of the concentrated waste stream (reject) in order to dispose of the arsenic contaminant. Therefore, membrane processes are costly as a method for removing arsenic species from an aqueous medium.
The present invention employs modified zeolite minerals as a method for removing arsenic species from an aqueous medium. Zeolite minerals are inexpensive minerals composed of crystalline hydrated aluminosilicates of group I and II metals in the periodic table. The isomorphous substitution of aluminum ions for silica ions into the component polyhedra causes a residual charge on the oxygen framework. Zeolites have a generally open framework which contain channels that can accommodate water molecules, and the cations necessary for charge balancing.
As stated hereinabove, zeolite minerals are very inexpensive compared to synthetic resins, and are readily available. Hence it is not economically necessary to remove adsorbed arsenic from them and reuse them. Further, the present invention for a method of removing arsenic species from an aqueous medium using modified zeolite minerals can remove arsenic from an aqueous medium to 50 ppb or less within about 2 hours of treatment.
Hence, one object of the present invention is to modify naturally occurring zeolite minerals with a concentrated ferrous aqueous solution in order to increase the zeolite mineral's affinity for aqueous arsenic species in an aqueous medium.
Therefore, the primary object of the present invention is to provide a method of removing arsenic species in the form of both arsenate and arsenite from an aqueous medium with modified zeolite minerals that does not possess the shortcomings of the prior art and offers the advantages of being able to achieve the removal of arsenic in the form of both arsenate and arsenite and is less expensive to use than the methods disclosed in the prior art.
Another object of the present invention is provide a method of removing aqueous arsenic species from an aqueous medium to a detection level for arsenic species of 5 ppb.
Yet another object of the present invention is to provide an inexpensive sorbent material to remove aqueous arsenic species from an aqueous medium which does not need to be reused in order to be economically applicable.
Yet still another object of the present invention is to provide an inexpensive sorbent material to remove aqueous arsenic species from an aqueous medium which will not leach aqueous arsenic species and can be readily disposed of as non-hazardous waste.
Another object of the present invention is provide a method of removing aqueous arsenic species from natural water.
Yet another object of the present invention is provide a method of removing aqueous arsenic species from natural water having a pH range from 5 to 8.
A further object is to provide a method of removing arsenic species from an aqueous medium using modified zeolite minerals comprising: providing an aqueous medium containing arsenic species in the form of both arsenate and arsenite; contacting the aqueous medium with an iron (II) laden zeolite mineral so that arsenic in the form of both arsenate and arsenite contained in the aqueous medium can be adsorbed onto the iron (II) laden zeolite mineral forming an arsenic adsorbed iron (II) laden zeolite mineral; and separating the arsenic adsorbed iron (II) laden zeolite mineral from the aqueous medium.
The foregoing has outlined some of the pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a more comprehensive understanding of the invention may be obtained by referring to the summary of the invention, and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims.